I am planning to bundle RPi along with a additional USB ports which has enough power to be used with External HDD. But I don't want the user to use two adapter. I want both to run from one.
What options do I have?
One option would be to power the USB hub and then power the RPi from the hub, thereby eradicating the need of having separate adapters.
Otherwise, you might have to create your own adapter.
Contrary to the advice often found on this site, I have successfully managed to run low power consumption HDD directly from RasPi USB port without any hubs for a few months without any problem.
The models you might be looking for are WD Green/Blue series, mine is WD10JPVT -- 1TB HDD with .55A peak consumption, that draws hardly more than half of that after the startup.
Just be sure to get 2A power supply and don't connect anything else to the USB.